Your pH is fine for any of the fish you've listed...it's more than close enough and they're not notoriously sensitive fish. The Firemouth actually will be fine between about 6.8 to 8 pH.
Severums don't need sand. They might like it, but it's not a need. THey should be fine with gravel.
However, I'd not go with angels in your tank. They need more height unless you plan to move them over to the 45 gallon tall when it's bigger.
A 30 long is a fun sized tank (I have a 33 gallon 36" long tank - same footprint just an inch or two taller). A single firemouth, severum or BP should do well in there (the firemouth would be OK forever). Lots of smaller cichlids (dwarfs like curviceps, bolivian rams, etc. up to keyholes) will do good in pairs or small groups in your tank.
Have fun!
Eric